Tools supplied with direct current are defined, for example, as magnetic plates that can be arranged on excavators or also mobile welding means.
Such generators are used mainly on material loading and unloading equipment on scrap yards or on demolition machines for recycling steel between concrete parts, where magnetic objects are moved by means of the magnetic plates mentioned in the introduction on the boom of an excavator or of a crane. Other machines, such as wheel mounts, wheel loaders, forklifts, and traveling cranes are also possible for such use.
Such magnetic systems are used in very harsh environments exposed to weather. Little consideration is also shown in practice for mechanical wear or damage to the generator as well as for the cabling thereof to the magnetic plate and the magnetic plate itself. The generator is usually located well protected in or around the engine area of the construction equipment. The cabling is embodied with commercially available multiwire lines from the generator to the magnetic plate via the arm of the excavator, via a helix cable from the forklift to the magnetic plate or other lines, which are more or less protected from mechanical destruction. The electric terminal of the magnetic plate is usually a 2-pole, 3-pole, or 5-pole junction box. This is usually located in the vicinity of the magnetic plate. The magnetic plate is used under harsh conditions. In blind areas, e.g., in ship or truck containers or in loading bays, the magnetic plate cannot always be placed carefully on the material to be loaded. Hard blows and shocks are common. It is also used at times incorrectly to compact material when loading containers. The cable, junction box or magnetic plate is frequently damaged due to the harsh use.
Electronically controlled generators are usually resistant to short-circuits and load failure. If the line is torn off completely during operation, a very high voltage will build up due to the inductance of the magnetic plate, and this voltage remains ignited via an arc at the torn-off cable until the energy of the magnetic plate drops in the arc below the ignition voltage of the arc.
The electronic control unit of the generator records this, as a rule, by the absence of output current and can respond to the failure by switching off the device. The live output lines of the electronically controlled generator are usually protected against overvoltage at the direct voltage output and on the life output lines, because the lines must reduce the inductive voltage of the magnetic plate during normal operation anyway.
If a short-circuit develops, the output lines are consequently short-circuited, e.g., by a pinched cable, and this is usually recognized in the electronically controlled generator by a current measurement. The electronic unit switches off in this case as well.
A brief explanation of the generator network in construction equipment shall be given below for the better understanding of the problem encountered in the state of the art.
Fuse disconnection is used, as a rule, in generators in construction equipment, i.e., the live wires are insulated against ground/ground wire (PE—protective earth). The use of an earth leakage monitor, which can recognize poor insulation against ground/PE is possible in case of this network only.
An unacceptably high voltage may build up between these potentials due to the separation of live wires and ground/PE. If this happens, a flashover of the voltage may occur on undefined components of the generator. This flashover usually destroys the electronic unit or parts thereof.
An unacceptably high voltage may develop on the generator system in the following exemplary cases, and even an earth leakage monitor integrated in the system may offer only limited protection at best in this case:                The line to the magnetic plate is torn off only partly and the torn-off cable of the magnetic plate comes into connection with the chassis of the magnetic plate, excavator arm or ground.        The junction box to the magnetic plate is mechanically destroyed or has water damage in connection with an interruption of only one line.        The magnetic plate has an internal short to ground in connection with an interruption of only one line.        The magnetic plate has poor insulation against ground/PE in connection with the interruption of only one line.        
The above-mentioned protective action of an earth leakage monitor can be explained as follows for completeness' sake on the basis of two errors:                The first error is, e.g., a poor insulation or short to ground/PE.        The second error is an interruption of a line from the electronically controlled generator to the magnetic plate. Should a suitable earth leakage monitor be used in the electronically controlled generator, this offers only a limited protective effect. It would offer protection only if the electronically controlled generator is switched off immediately in the first error (insulation value<23 kOhms) and the electronically controlled generator is blocked by means of a reclosure preventing device. The earth leakage monitor offers no protection if the first error is above the threshold of typically 23 kOhms.        
There also is no significant protection if the second error occurs, without switching off the electronically controlled generator, or the second error occurs with a short time difference from the first error.
US 2005/0094345 A1 discloses a protective means in the form of an insulating spark gap between the two feed lines of an induction means in the form of a magnetic coil on an excavator. This protective means is used for protection in case of failure of a feed line based on the harsh ambient conditions occurring in case of such construction equipment.
US 2009/0160590 A1 shows a process and a device for controlling a lifting magnet, which is supplied via an alternating-voltage generator with downstream rectifier bridges.
DE 10 2007 015 933 A1 discloses an overvoltage protection means for use in direct-current networks, in which a bypass with a spark gap is again provided.
Similar spark gaps for protective electric devices can be found in DE 10 2008 025 501 A1 and DE 44 27 032 C1.
DE 28 51 537 A1 shows a protective means, for example, for high-power transmitters with a spark gap and protective resistor, in which the spark gap is scanned by means of an optical sensor and whose response is detected for quickly disconnecting the power supply.